The bevelled edge

A ruler's markings sit on top of a certain thickness of material, and that thickness is the gap between the number you read and the surface you are actually measuring against. A bevelled edge shaves that gap down, and the difference shows up as a small but real reduction in one specific source of error.

What a bevel does

Most rulers are cut square: the edge is a flat rectangle in cross-section, and the printed scale sits on the flat top face, a millimetre or two above whatever the edge is touching. A bevelled-edge ruler is ground at an angle along the scale side, so the printed marks sit much closer to the point of contact - sometimes almost at it. The scale is, in effect, brought down to the surface rather than held above it on a slab of plastic or steel.

Why the gap matters

Reading a mark that sits above the surface it is measuring means sighting down and across a small distance, and any deviation from looking exactly straight down introduces parallax - the mark appears to line up with a different point than it actually does, in a direction that depends on where your eye is. This is the same effect a viewing-angle problem produces when reading your own ruler from an awkward position, and a bevelled edge does not remove the need for good viewing technique - it just shrinks the vertical distance parallax has to work with, so the same small angling error produces a smaller mistake.

Whether it is worth seeking out

For length, where the ruler is pressed to bone and read at the tip, a bevel is a genuine but modest improvement - useful, not essential, since careful sighting on a square-edged ruler still gets you close. A square-edged ruler with a true zero and clear millimetre marks passes the accuracy bar this site cares about on its own; a bevel is worth choosing between two otherwise-equal rulers, not worth buying a new one for. It has nothing to do with a rating from a photograph, where an image model is inferring a figure rather than sighting down a physical edge at all, and nothing to do with a score from a rating tool, which is a position in a distribution of images rather than a reading taken against any edge. A person's stated opinion has no edge to sight along either - an honest human impression is a judgement, not a reading - and neither does a rating from Rate Cock, which was never claiming to be a physical measurement in the first place.